Method for the production of synthetic silica glass

ABSTRACT

The invention relates to a previously known method for producing synthetic silica glass, comprising the following steps: a gas stream containing a vaporizable initial substance, which can be converted into SiO 2  by means of oxidation or flame hydrolysis, is formed; the gas stream is delivered to a reaction zone in which the initial substance is converted so as to form amorphous SiO 2  particles; the amorphous SiO 2  particles are deposited on a support so as to form an SiO 2  layer; and the SiO 2  is vitrified during or following deposition of the SiO 2  particles in order to obtain the silica glass. The aim of the invention is to create an economical method for producing synthetic silica glass, which is characterized by a favorable damaging behavior towards short-wave UV radiation while being particularly suitable for producing an optical component used for transmitting high-energy ultraviolet radiation having a wavelength of 250 nm or less. Said aim is achieved by using a mixture of a monomeric silicon compound containing a singular Si atom and an oligomeric silicon compound containing several Si atoms as an initial substance, provided that the oligomeric silicon compound in the mixture contributes less than 70 percent to the total silicon content.

The present invention relates to a method for producing synthetic silicaglass, comprising the steps of:

-   -   a) forming a gas stream containing a vaporizable initial        substance which can be converted into SiO₂ by means of oxidation        or flame hydrolysis,    -   b) supplying the gas stream to a reaction zone in which the        initial substance is converted so as to form amorphous SiO₂        particles,    -   c) depositing the amorphous SiO₂ particles on a support so as to        form an SiO₂ layer,    -   d) vitrifying the SiO₂ layer either during or following        deposition of the SiO₂ particles to obtain the silica glass.

Such methods for producing synthetic silica glass by oxidation of flamehydrolysis of silicon-containing initial substances are generally knownunder the names VAD method (vapor phase axial deposition), OVD method(outside vapor phase deposition), MCVD method (modified chemical vapordeposition) and PCVD method (or also PECVD method; plasma enhancedchemical vapor deposition). In all of these methods, SiO₂ particles arenormally produced by means of a burner and deposited in layers on asupport which is moved relative to a reaction zone. At an adequatelyhigh temperature in the area of the support surface, the SiO₂ particlesare vitrified immediately (“direct vitrification”). By contrast, in theso-called “soot method” the temperature is so low during deposition ofthe SiO₂ particles that a porous soot layer is obtained that is sinteredin a separate process step to obtain transparent silica glass. Bothdirect vitrification and soot method yield a dense, transparentsynthetic silica glass of high purity.

The support is normally removed in a subsequent process step. Quartzglass blanks are thereby obtained in the form of rods, blocks, tubes orplates which are further processed into optical components, particularlylenses, windows, filters, mask plates, for use in microlithography.

A useful initial substance for producing synthetic silica glass issilicon tetrachloride (SiCl₄). However, many other silicon-organiccompounds have also been suggested from which SiO₂ can be formed byhydrolysis or oxidation. As examples of suitable initial substances andas literature, the following should here be indicated:

Monosilane (SiH₄; DE-C 38 35 208), alkoxysilanes (R_(4-n) Si(OH)_(n),where R represents an alkoxy group having one to four C atoms), andnitrogen silicon compounds in the form of silazanes (EP-A 529 189). Theso-called polysiloxanes (also abbreviated as “siloxanes”) formparticularly interesting initial substances, and their use for producingsynthetic SiO₂ is for example suggested in DE-A1 30 16 010 and in EP-B1463 045. The substance group of the siloxanes can be subdivided intoopen-chain polysiloxanes (chain polysiloxanes for short) and intoclosed-chain polysiloxanes (cyclopolysiloxanes for short). The chainpolysiloxanes are described by the following chemical formula:R₃Si·(SiR₂O)_(n)·SiR₃where n is an integer≧0. The cyclopolysiloxanes have the followinggeneral formula:Si_(p)O_(p)(R)_(2P)where P is an integer≧2. The residue “R” is each time for example analkyl group, preferably a methyl group.

The optical components made from the synthetic silica glass are interalia used for transmitting high-energy ultraviolet radiation, e.g. inthe form of optical fibers or as optical exposure and projection meansin microlithography devices for producing large-scale integratedcircuits for semiconductor chips. The exposure and projection systems ofmodern microlithography devices are equipped with excimer lasers thatemit high-energy pulsed UV radiation of a wavelength of 248 nm (KrFlaser) or of 193 nm (ArF laser).

Short-wave UV radiation of this type may produce absorption-inducingdefects in optical components of synthetic silica glass. Type and extentof a defect formation depend on the type and quality of thecorresponding silica glass which are substantially determined bystructural properties, such as density, refractive index profile,homogeneity and chemical composition.

The influence of the chemical composition of synthetic silica glass ondamage behavior upon irradiation with high-energy UV light is e.g.described in EP-A1 401 845, which also discloses a generic productionmethod. Hence, high radiation resistance is achieved in a silica glasswhich is characterized by high purity, an OH content ranging from 100 wtppm to about 1,000 wt ppm and, at the same time, by a relatively highhydrogen concentration of at least 5×10¹⁶ molecules/cm³ (based on thevolume of the silica glass).

In the damage patterns described in the literature, a distinction can bemade between those patterns in which an increasing absorption isobserved during continuous UV irradiation (induced absorption) and thosepatterns in which structural defects are produced in the glassstructure, such defects being e.g. manifested by fluorescence generationor by a change in the refractive index, which however need notnecessarily change the radiation absorption.

In the damage patterns of the first group the induced absorption maye.g. rise linearly, or saturation is accomplished after an initial rise.Furthermore, it is observed that an initially existing absorption bandwill disappear within a few minutes after the UV source has beenswitched off, but will soon regain the former level after renewed startof the irradiation process. The last-mentioned behavior is called “rapiddamage process” (RPD) in the literature. Furthermore, a damage patternis known where structural defects evidently accumulate in the silicaglass such that these manifest themselves in a sudden strong increase inabsorption. The strong increase in absorption is called “SAT defect” inthe literature.

In connection with the damage patterns of the second group, a knownphenomenon is the so-called “compaction” which occurs during or afterlaser irradiation with a high energy density. This effect manifestsitself in a local density increase which leads to a rise in therefractive index and thus to a deterioration of the imaging propertiesof the optical component. An opposite effect is observed when an opticalcomponent made of silica glass is subjected to laser radiation of a lowenergy density but with a high pulse number. These conditions willcreate so-called “decompaction”, which is accompanied by a decrease inthe refractive index. Irradiation will also lead to a local densitychange and thus to a deterioration of the imaging properties. Compactionand decompaction are thus also defects that may limit the service lifeof an optical component.

It is therefore the object of the present invention to provide aneconomic method for producing synthetic silica glass that ischaracterized by a favorable damage behavior with respect to short-waveUV radiation and that is particularly suited for producing an opticalcomponent for transmitting high-energy ultraviolet radiation of awavelength of 250 nm or less.

Starting from the above-mentioned method, this object is achievedaccording to the invention in that a mixture of a monomeric siliconcompound containing a singular Si atom and of an oligomeric siliconcompound containing several Si atoms is used as the initial substance,with the proviso that the oligomeric silicon compound in the mixturecontributes less than 70% to the total silicon content.

In contrast to the known methods in which an initial substance is usedthat normally consists of a single and defined silicon compound which isas pure as possible, the present invention suggests the use of a mixtureof several silicon compounds, with the proviso that one of the siliconcompounds should be one containing a singular Si atom (hereinaftercalled “monomeric silicon compound” or “monomer” for short) and thatanother one of the silicon compounds should be one containing several Siatoms (hereinafter called oligomeric silicon compound or “oligomer” forshort).

In the oligomeric silicon compound two or more silicon atoms are bondedto each other via one or several oxygen bridges. A typical examplethereof are siloxanes.

Depending on the number of the silicon atoms in the silicon compound,these “oligomers” will hereinafter also specifically be called “dimers”in the case of two silicon atoms and “trimers” in the case of threesilicon atoms.

When start material is used in the form of a monomeric silicon compound,a silica glass is obtained that shows high radiation resistance toshort-wave UV laser radiation. This is particularly manifested by a hightransmission of the silica glass, a low saturation level of the inducedabsorption and hardly any proneness to compaction or decompaction at thelaser-radiation energy densities which are typical of microlithography.

By contrast, it has been found that synthetic silica glass which hasbeen produced by using an oligomer, particularly an oligomer having ahigh amount of ring structures, shows increased defect formationvis-á-vis short-wave UV laser radiation. Therefore, this silica glassquality shows a comparatively low radiation resistance especially at thelaser-radiation energy densities typical of microlithography, which isparticularly manifested by a higher saturation level of the inducedabsorption. Moreover, it has been found that in such a silica glass theso-called “homogenization”, in which a glass item is repeatedly twistedin different directions, requires more efforts than in a silica glassproduced by using SiCl₄.

These observations suggest that the structure of the SiO₂ networkobtained during glass production depends on the initial substance used.A possible explanation for this could be that because of the closevicinity of the silicon atoms in an oligomer a comparatively large partof the SiO₂ primary particles formed during oxidation or hydrolysis iscomposed of two or more silicon atoms, said SiO₂ primary particlesgrowing in the reaction zone into larger SiO₂ particles, e.g. bycoagulation or condensation.

By contrast, the SiO₂ particles in a monomeric silicon compound (e.g.alkoxysilanes, alkylsilanes, SiCl₄) are formed by oxidation orhydrolysis of individual molecules, each containing only one siliconatom. Hence, it must be assumed that a large part of the SiO₂ primaryparticles initially formed in the reaction zone contain only one siliconatom.

During agglomeration into larger SiO₂ particles the SiO₂ primaryparticles formed in this way show a behavior differing from that of theSiO₂ primary particles produced from oligomers. In oligomeric siliconcompounds, depending on their stoichiometry, more dimeric or oligomericSiO₂ primary particles are present than during the conversion ofmonomeric silicon compounds. Depending on the number and configurationof the silicon atoms in the initial substances, the size of the primaryparticles and thus also the size of the resulting SiO₂ particles and theconcentration thereof in the reaction zone will change. Moreover, thisparameter also has an effect on the temperature within the reaction zoneand thus on the whole deposition process in such a way that in anoligomer a network structure is obtained that shows the above-mentioneddrawbacks with respect to radiation resistance.

On the other hand it is known that a higher deposition rate is achievedin the deposition process using an oligomeric silicon compound. Theproduction method is thus more economic, which is even promoted by thefact that the oligomeric silicon compound based on the silicon contentis less expensive than a monomeric silicon compound.

Surprisingly, it has now been found that the use of an initial substancein the form of a mixture containing at least one monomeric siliconcompound and at least one oligomeric silicon compound can yield a silicaglass having a radiation resistance comparable to that of a silica glassproduced from a monomeric silicon compound. A precondition is howeverthat the silicon amount deriving from the oligomeric silicon compoundsin the mixture accounts for less than 70% of the total silicon contentof the mixture.

Mixing of the different silicon compounds can basically be performed atany process stage. Mixing in the liquid phase presupposes that there areno reactions between the components that impair vaporization or reactionin the reaction zone.

This is e.g. often the case with mixtures of chlorine-containing andchlorine-free silicon compounds when polymerization reactions takeplace. Due to these observations mixing is preferably carried out in thegas phase and, if possible, at a late process stage, so that at leasttwo vaporizer systems are needed as a rule. It is also possible that thesilicon compounds are not intermixed before the reaction zone in thatthey are separately supplied to the reaction zone.

It is thereby possible to produce a silica glass in the case of whichthe efficiency of the production method is improved due to the use ofoligomeric silicon compounds, and whose homogenizability and radiationresistance (with respect to its induced absorption and its behavior withrespect to compaction and decompaction) do not substantially differ,despite the use of oligomeric silicon compounds, from a silica glassproduced from monomeric silicon compounds.

It has turned out to be advantageous when the oligomeric siliconcompound in the mixture contributes less than 60% to the total siliconcontent.

The smaller the amount deriving from the oligomeric silicon compound isin the total silicon demand, the better will be the resulting silicaglass with respect to its homogenizability and radiation resistance. Acontribution of less than 60% to the total silicon content has turnedout be a particularly helpful compromise between radiation resistanceand homogenizability of the silica glass on the one hand and theefficiency of the method on the other hand.

However, when the amounts of the oligomeric silicon compound are verysmall, its contribution to an enhanced efficiency of the method will nolonger be noticed. Therefore, the oligomeric silicon compound in themixture preferably contributes at least 30% to the total siliconcontent.

Due to their efficiency ring-like oligomers are preferably used. The useof an oligomeric silicon compound in the form of a polyalkylsiloxane hasturned out to be particularly advantageous.

Polysiloxanes are characterized by a particularly high amount of siliconper weight, which contributes to the efficiency of the method. Forinstance, the weight portion of silicon in(octamethylcyclotetrasiloxane) OMCTS and in(decamethylcyclopentasiloxane) DMCPS is 37.9% each time, and inhexamethyidisiloxane it is 34.6%.

For this reason, and because of its large-scale availability togetherwith a high purity, the polyalkylsiloxane which is preferably used inthe method of the invention is an octamethylcyclotetrasiloxane (OMCTS)or a decamethylcyclopentasiloxane (DMCPS).

Alternatively, it has also turned out to be advantageous when achlorine-free alkoxysilane is used as the monomeric silicon compound.

Alkoxysilanes are also characterized by large-scale availability andhigh purity. The absence of chlorine may have an advantageous effect onradiation resistance.

With respect thereto the use of an alkoxysilane in the form ofmethyltrimethoxysilane (MTMS) or a tetramethoxysilane (TMS) isparticularly preferred.

The use of MTMS for silica glass production has the additional advantagethat it is hardly toxic.

As for its large-scale availability and purity, silicon tetrachloride(SiCl₄) is advantageously used as the monomeric silicon compound.

As for the radiation resistance of the silica glass, a procedure hasturned out to be particularly advantageous in which a mixture is used inwhich the ratio of the mixing amounts of MTMS and OMCTS is in the rangeof 40:60 to 60:40, preferably around 45:55 (based on the molecularsilicon amount).

The mixing ratio refers to the respective amounts of the substances inthe gas phase in which the substances are present in vaporized form. Forsetting a mixing ratio of 45:55 a gravimetric mixing ratio of MTMS toOMCTS of about 1.5:1 must be set.

In another procedure using SiCl₄ as the monomeric silicon compound, ithas turned out to be useful when a mixture is employed in which theratio of the mixing amounts of SiCl₄ and OMCTS, based on the molecularsilicon amount, is between 30:70 and 70:30.

In a silica glass which is exclusively produced by using SiCl₄, achlorine content ranging from 60 wt ppm to 130 wt ppm is normallymeasured. Due to mixing of a chlorine-free component (such as OMCTS) andthe chlorine-containing component SiCl₄ a chlorine content of less than60 wt ppm, but of more than about 10 ppm, can be adjusted in the silicaglass in an easy way.

It has been found that in such a silica glass the damage mechanismsleading to compaction and decompaction are avoided or at least reducedconsiderably. Changes in the refractive index in the course of theintended use of components made from silica glass are avoided eithercompletely or to a large degree, so that the said damage mechanisms donot limit the service of the optical components made from the silicaglass.

Preferably, a chlorine-free silicon compound is used as the oligomericsilicon compound.

Hence, even if a chlorine-containing monomeric silicon compound is usedin the mixture, a silica glass can be produced that has a low chlorinecontent and turns out to be superior particularly with respect to thedamage patterns known as compaction/decompaction.

The silicon compounds can basically be mixed in a liquid phase or in agaseous phase. However, a procedure is preferred in which the siliconcompounds are vaporized separated from each other, the mixture beingproduced before or during method step b), e.g. before the gas stream isfed into the reaction zone.

This pre-mixing ensures a defined composition of the gas stream duringintroduction into the reaction zone and thus a reproducible and definedreaction sequence.

The present invention shall now be explained in more detail withreference to embodiments. As the sole figure,

FIG. 1 shows a variant of the method according to the invention forproducing an SiO₂ soot body.

In the apparatus shown in FIG. 1, there is provided a support tube 1consisting of aluminum oxide, along which several series-arranged flamehydrolysis burners 2 are arranged. The flame hydrolysis burners 2 aremounted on a joint burner block 3 which can be reciprocated in parallelwith the longitudinal axis 4 of the support tube 1 and is displaceablein a direction perpendicular thereto, as outlined by directional arrows5 and 6. The burners consist of silica glass; their distance from oneanother is 15 cm.

Each of the flame hydrolysis burners 2 has assigned thereto a burnerflame 7 having a main propagation direction 8 perpendicular to thelongitudinal axis 4 of the support tube 1. A control device 9 which isconnected to a drive 10 for the burner block 3 is provided forcontrolling the movement of the burner block 3.

With the help of the flame hydrolysis burners 2, SiO₂ particles aredeposited on the support tube 1 which is rotating about its longitudinalaxis 4, so that the blank 11 is built up in layers. To this end theburner block 5 is reciprocated along the longitudinal axis 4 of thesupport tube 1 between two reversal points that are stationary relativeto the longitudinal axis 4. The amplitude of the reciprocating movementis marked by directional arrow 5. It is 15 cm, thus corresponding to theaxial distance between the burners 2. In the deposition process atemperature of about 1200° C. is accomplished on a surface 12 of theblank.

The flame hydrolysis burners 2 are each supplied with oxygen andhydrogen as burner gases and with a gaseous mixture of chlorine-freeinitial substances as the initial material for the formation of SiO₂particles.

After the deposition process has been completed, a soot tube is obtainedthat is subjected to a dehydration treatment and is vitrified so as toform a silica glass tube. A round rod which is free from striae in threedimensions and has a diameter of 80 mm and a length of about 800 mm isproduced from the silica glass tube by repeated twisting at temperaturesof about 2000° C. in different directions (homogenization). The behaviorof the silica glass during homogenization is recorded each time.

Using heat deformation at a temperature of 1700° C. and anitrogen-flushed melt mold, a circular silica glass block is formedtherefrom with an outer diameter of 300 mm and a length of 90 mm.

For eliminating stress birefringence the silica glass block obtained inthis way is subsequently subjected to a standard annealing treatment asdescribed in EP-A1 401 845. To this end the silica glass block isheated, inter alia in air and at atmospheric pressure, to 1100° C. andis subsequently cooled at a cooling rate of 1° C./h. A stressbirefringence of not more than 2 nm/cm is measured. The mean OH contentis about 900 wt ppm. The silica glass block produced in this way isimmediately suited as a blank for producing an optical lens for amicrolithography device. For measuring the damage behavior of the silicaclass cylindrical measurement samples are cut having the dimensions 10mm×10 mm×40 mm, and each of their four long sides is polished. Fordetermining the radiation resistance the measurement samples are eachirradiated by an UV excimer laser (wavelength=193 nm, pulse energy=100mJ/cm², pulse repetition rate=200 Hz), the transmission beingsimultaneously measured at a wavelength λ=193 nm. Moreover, the behaviorof the silica glass with respect to its compaction and decompactionbehavior was determined, as described in “C. K. Van Peski, R. Morton andZ. Bor (“Behaviour of fused silica irradiated by low level 193 nmexcimer laser for tens of billions of pulses”, J. Non-Cryst. Solids 265(2000) pp. 285-289).

Table 1 shows the homogenizability and radiation resistance determinedon the produced silica glass for different initial substances and mixingratios, and it indicates the efficiency of the respective manufacturingmethod in terms of quality. TABLE 1 Radiation Radiation MonomericOligomeric resistance resistance Si Si Mixing Homo- (induced (compac./No. compound compound ratio genizability absorption decompac. Efficiency1 SiCl₄ — — ++ ++ + 0 2 MTMS OMCTS 45:55 ++ ++ ++ + 3 MTMS HDMS 45:55 +++ ++ + 4 SiCl₄ OMCTS 45:55 ++ ++ ++ + 5 MTMS OMCTS 25:75 − 0 ++In the table,MTMS stands for methyltrimethoxysilane,OMCTS stands for octamethylcyclotetrasiloxane,HDMS stands for hexamethylcyclotetrasiloxane.

The figures of the mixing ratios of the samples designate the amountderiving from the respective substances in the total silicon content ofthe silica glass. For instance, in sample no. 1 the silicon amountderived from MTMS covers 45% of the total silicon demand and the siliconfrom the OMCTS contributes 55% thereto.

The qualitative results in Table 1 show that the use of an initialsubstance in the form of a mixture containing a monomeric siliconcompound and an oligomeric silicon compound yields a silica glass in aneconomic way that has a radiation resistance comparable with a silicaglass produced from a monomeric silicon compound. With an increasingamount of the oligomeric silicon compound in the mixture, the efficiencyof the silica-glass production process increases and radiationresistance and homogenizability of the silica glass decrease. If the Siamount of the silica glass derives from the oligomeric silicon compoundat not more than 70%, radiation resistance and homogenizability areadequate after all.

A similar result is obtained when instead of the soot method the silicaglass is produced by direct vitrification.

1. A method for producing synthetic silica glass, said method comprising the steps of: forming a gas stream containing a vaporizable initial substance which can be converted into SiO₂ by means of oxidation or flame hydrolysis, supplying the gas stream to a reaction zone in which the initial substance is converted so as to form amorphous SiO₂ particles, depositing the amorphous SiO₂ particles on a support so as to form an SiO₂ layer, vitrifying the SiO₂ layer either during or following deposition of the SiO₂ particles to obtain the silica glass, wherein the initial substance comprises a mixture of a monomeric silicon compound containing no more than one Si atom per molecule thereof and of an oligomeric silicon compound containing a plurality of Si atoms in each molecule thereof the silicon in the oligomeric silicon compound in the mixture constituting less than 70% of a total silicon content of the initial substance.
 2. The method according to claim 1, wherein the silicon in the oligomeric silicon compound in the mixture constitutes less than 60% to the total silicon content.
 3. The method according to claim 1, wherein the silicon in the oligomeric silicon compound in the mixture constitutes at least 30% to the total silicon content.
 4. The method according claim 1, wherein the oligomeric silicon compound is a polyalkylsiloxane.
 5. The method according to claim 4, wherein the polyalkylsiloxane is an octamethylcyclotetrasiloxane (OMCTS) or a decamethylcyclopentasiloxane (DMCPS).
 6. The method according to claim 1, wherein the monomeric silicon compound is a chlorine-free alkoxysilane.
 7. The method according to claim 6, wherein the alkoxysilane is methyltrimethoxysilane (MTMS) or a tetramethoxysilane (TMS).
 8. The method according to claim 1, wherein the monomeric silicon compound is silicon tetrachloride (SiCl₄).
 9. The method according to claim 1, wherein the oligomeric silicon compound is an octamethylcyclotetrasiloxane (OMCTS) and the monomeric silicon compound is methyltrimethoxysilane (MTMS); the mixture having MTMS and OMCTS therein in respective mixing amounts such that a ratio of the mixing amounts of MTMS and OMCTS, based on a molecular silicon amount thereof, is in the range of 40:60 to 60:40.
 10. The method according to claim 1, wherein the oligomeric silicon compound is an octamethylcyclotetrasiloxane (OMCTS) and the monomeric silicon compound is silicon tetrachloride (SiCl₄); and the mixture having SiCl₄ and OMCTS therein in respective mixing amounts such that a ratio of the mixing amounts of SiCl₄ and OMCTS, based on a molecular silicon amount thereof, is between 30:70 and 70:30.
 11. The method according to claim 1, wherein the oligomeric silicon compound is a chlorine-free silicon compound.
 12. The method according to claim 1, wherein the silicon compounds are vaporized separated from each other and that the mixture is produced before or during the step of supplying the gas stream to the reaction zone.
 13. The method according to claim 9, wherein the ratio of the mixing amounts of MTMS and OMCTS is approximately 45:55. 